Apparatus for machining electrically conducting substances by electrochemical attack

ABSTRACT

Apparatus for etching fine grooves in surfaces by the combination of pulsed currents in a modified Electro-Chemical Machining method in combination with a flow guiding template. 
     The apparatus is very suitable for the etching of grooves in a surface used in a bearing.

This is a division, of application Ser. No. 705,174 filed July 14, l976,now U.S. Pat. No. 4,073,710.

The invention relates to a method of removing material from electricallyconducting substances by electrochemical attack resulting from thepassage of current between a workpiece of this material and an electrodeduring the afflux of electrolyte-containing liquid.

With the use of this methode, the charge of the material is almostalways positive, while that of the electrode is negative.

The above method is used especially for the machining of so-calleddifficultly machinable materials and surfaces. An abbreviation for themethod is E.C.M. (Electro Chemical Machining).

A detailed description of the E.C.M. process is given in the journalScientific American, January 1974, pp. 30-37.

A drawback of the E.C.M. process is that the rate of flow of theelectrolyte along the material to be machined must be high. Thefollowing is stated about this in the middle of page 35:

"Another reason for keeping the rate of flow high is to remove corrosionproducts from the gap. Between workpiece and electrode. If theelectrolyte -containing liquid becomes contaminated with grains of metalor other substances, the rate of flow becomes uneven, the distributionof current becomes nonuniform and the surface finish is poor. To avoidthis problem it is best to fit the flow line of the electrolyte with afilter."

It has been found in practice, however, that the use of a filter in theflow of electrolytic liquid which is recirculated in this case does notconstitute a satisfactory solution. The corrosion products are indeedformed directly at the point where the electrochemical attack takesplace, where they interfere with the process. This reduces thepossibility of rapidly obtaining fine grooves in surfaces that aresuitable for the assembly of bearings.

The invention aims at obviating the aforementioned drawback, as that theprior-art E.C.M. process unexpectedly receives a broader field ofapplications.

According to the invention, the passage of current is pulsed. Since thepassage of current is pulsed, the corrosion products are washed awayfrom the surface to be machined by the electrolyte-containing liquidduring the interval that no passage of current takes place.

The surface finish is greatly improved by the pulsed feed of current, sothat the E.C.M. method now becomes applicable for the removal of fairlysmall amounts of material. Surface deposits which could not be simplywashed away used to be an impediment to the application of the E.C.M.method in producing fine grooves in workpieces of hard steel.

In the past, the E.C.M. method tended to be more suitable for removinglarger amounts of material, where interference with the process bycorrosion products was not serious. A further advantage is that the rateof afflux of the electrolyte may be less high. Use is preferably made ofcurrent pulses ranging from 5 to 20 milliseconds. The number of pulsesfrom case to case is governed by the groove depth which is to beattained.

It is stated on pages 32 and 34, respectively, of the aforementionedpublication in the journal Scientific American that a watery solution ofsodium chlorate is used as electrolyte and that the distance betweenworkpiece and electrode ranges from about 5 to 150 microns for removingrelatively large amounts of material.

With the use of these known data, it is possible to obtain fine groovesby means of a pulsed passage of current, for example if the solutioncontains from 5 to 40 percent by weight of sodium chlorate and theapplied voltage is comprised between 10 and 50 volts.

The amount of material removed and the groove depth related thereto canbe controlled by integrating the current pulses applied.

It is possible to adjust the distance and the uniformity of the distancebetween the electrode and the workpiece by reversing the polarity of theelectrode. During the passage of current, the electrode is thenattacked, instead of the workpiece, causing the distance between theworkpiece and the electrode to be increased. The electrode, which isnormally negative, then becomes positive. It is to be noted that thisadjustment involves very small distances and can only be satisfactorilyperformed with the use of pulsed passage of current. The removal of asmall amount of material by the input of a very small amount of powercan be better controlled by a pulsed current than is the case with anonpulsed passage of current.

A method is known from Netherlands Pat. application No. 72-05,289according to which a flow-guiding template is used in causing a liquidto flow along a workpiece at points determined by this template, thetemplate being adapted in shape to the surface to be treated and beingprovided with channels for the afflux of liquid at the points whereelongate recesses or grooves are desired in the workpiece.

This prior-art method deals with providing grooves by etching. Thegroove depth is here governed by the flow conditions in the affluxchannels. A more deeply incised channel in the template itself and ahigher flow velocity of the etching liquid along the workpiece thenresult in a deeper groove in the workpiece.

This known method can be improved, while preserving the exactness of thegroove pattern to be obtained by etching, by providing one or moreelectrodes in the form of the desired recesses in the afflux channelsthat are embedded in the template. In this case, pulsed current isadmitted between electrodes and workpiece. As a result of thisimprovement, groove patterns can be obtained much more rapidly than bythis prior-art process.

Netherlands Pat. application No. 72-05,289 specifies templates ofelectrically nonconducting materials such as plastics or glass.

The afflux templates can be adapted both to flat and to curved surfaces.The flow in the afflux channels can be longitudinal as well asperpendicular as well as at an angle with respect to the surface to bemachined. In the electrochemical attack of material in which grooves areto be formed, use is preferably made of a method where the liquid flowis directed along the workpiece and along the electrodes in the channelsin the template, which direction of flow requires the lowest pressurefor propelling the electrolyte and yields a product the quality of whichcan be particularly well controlled.

The aforedescribed method can be implemented in a very wide variety ofmanners. It is possible to use a point-shaped electrode with respect towhich a flat or curved workpiece is moved during the attack. The curvedsurface can be constituted, for example, by a small round rod of metalwhich is rotated about its axis and at the same time performs a motionof translation in the direction of its axis. It is also possible toimpart to steel spheres provided with a pivot a rotary motion withrespect to a point-shaped electrode during the machining operation.

The invention can be implemented with an apparatus comprising some partsthat are known from Netherlands Pat. application No. 72-05,289. Thispatent application specifies an apparatus comprising a flow-guidingtemplate 1 which is provided with one or more liquid inlets 2, liquidoutlets 3 and afflux channels 4 inside the template, the form of thetemplate being adapted to a surface of the workpiece 5 which is to bemachined. In implementing the method according to the invention, thisprior-art apparatus is furthermore provided with a source of directcurrent 6, the positive pole of which is connected by way of a pulsegenerator 7 and a resister 10 -- which resistor 10 is connected inparallel to a current integrator 8 -- to electrodes 9 which are providedin the afflux channels 4 and are embedded in the flow-guiding template1, while a positive pole of the source of direct current 6 can at thesame time be connected to the workpiece 5.

Although the electrochemical method of removing material can be used fora wide variety of workpieces, it is preferably applied to surfaces whichcan be used in a bearing.

Without any restriction being intended, a description will be givenhereinafter of the diagram of the FIGURE.

The FIGURE relates to a system for removing material where the flow ofthe electrolyte is parallel both to the workpiece and to the electrodes.In the FIGURE, the number 1 indicates a flow-guiding template which inthis case is equipped with two liquid inlets 2, two liquid outlets 3 andtwo afflux channels 4 inside the template 1. The shape of the template 1is adapted to the surface of a workpiece 5 which is to be machined. Theafflux channels 4 contain two electrodes 9 which are embedded in thematerial of the template 1, which is here supposed to be electricallyinsulating. The electrodes 9 are connected to the negative pole of asource of direct current 6; the connection to the cource of directcurrent is established by way of a pulse generator 7, which only passesdirect-current pulses of 5 to 20 milliseconds, and of a resistor 10. Acurrent integrator 8, which collects and integrates the electric signalof the voltage drop along the resistor 10, is connected in parallel tothe resistor 10. The integrator 8 serves for process control, since thetotal quantity of current applied is a measure for the amount ofmaterial removed. The workpiece 5 is connected to the positive pole ofthe source of direct current 6.

It is to be noted that the direction of flow of the Electrolyte does nothave to be entirely parallel to the electrodes. The direction of flowcan be at an angle to the electrodes when the position of the electrodeis at an angle to the axis of symmetry of a curved surface which is tobe machined, so that the direction of the afflux channels cannot bedesigned parallel or perpendicular to the electrode.

Such a case arises in the machining of a cylindrical or spherical pivotfor a bearing.

As additional state of the art can be mentioned French Pat. No.1,417,497 (Mitsubishi), French Pat. No. 2,166,760 (Centralec), FrenchPat. No. 1,280,813 (Agie), U.S. Pat. No. 3,371,022 (Kihoshi Inoue),French Pat. No. 1,526,671 (Associated Engineering) and French Pat. No.1,565,759 (E. Pryor Developments).

However, from all these additional publications it was not obvious toderive a method for the very carefully etching of an intricate shallowgroove pattern on the surface of an electrical conducting work piecesuch as e.g. a steel ball, wherein a non-conducting template providedwith electrodes and pulsed current is used.

With respect to French Pat. No. 1,417,497 (Mitsubishi) there can bementioned three important differences with the new method. First thereare no real current pulses used, according to FIG. 16 a mechanical pulsesupplier is used or according to FIGS. 4 and 9 a rectified 50 Hzcurrent. Nowhere in French Pat. No. 1,417,497 the length of the pulsetime is given. For this reason this method cannot be applicated for theremoval of very little amounts of material. Secondly the afflux ofliquid is caused through the electrode itself and not by means of a flowguiding template, which method has the possibility of carefullydesigning the liquid inlets and outlets. A template with carefullydesigned inlets and outlets for liquid is necessary for obtaining therequired precision and equality of groove patterns required fortechnical purposes.

The last difference is the fact that reversing of the polarity accordingto the new method is effected only in order to adjust the distancebetween work piece and electrode. During the normal attacking processthe electrode is not consumed.

According to French Pat. No. 2,166,760 (Centralec) an isolating mask isapplied between the electrode and the work piece. This method is notsuitable for the manufacture of very intricate surface groove patternsas the isolating mask described has no possibility for carefullydesigning the afflux canals for liquid.

In the French Pat. No. 1,280,813 (Agie) integration of the tension isapplicated only in order to control the chemical attack itself of thework piece and not the total control of the manufacture of a groovepattern.

The process described in the U.S. Pat. No. 3,371,022 (Kihoshi Inoue)refers to a method for electro chemical machining, wherein greateramounts of materials are to be removed than the new method. Besidesthere are used two different kinds of liquids, because a liquiddielectric medium surrounds a liquid stream of electrolyte during theprocess. This method differs therefore wholly from the new process.

The resemblance between the new method and the method described inFrench Pat. No. 1,526,671 (Associated Engineering) is that the currentof an electro chemical machining device is measured in order todetermine the rate of electro chemical attack. Nowhere in thispublication the fact is mentioned that electrodes in combination with anafflux template are used.

According to French Pat. No. 1,565,759 (E. Pryor Developments) a flowguiding template and an electrode are known. As the specific use is onlythe electrolytic marking of metallic subjects, it was not obvious to usethis electrode and the flow guiding template with a pulsed directcurrent for the manufacture of intricate groove patterns.

I claim:
 1. In an apparatus for removing material from an electricallyconducting workpiece by electrochemical attack of the kind including anelectrically insulating flow-guiding template having afflux channels ina surface of the template, said channels being in communication with atleast one liquid inlet and at least one liquid outlet and said templatesurface being adapted to engage the surface of the workpiece, theimprovement which comprises electrodes disposed in said channels andembedded in the template, means for connecting the electrodes to thenegative pole of a direct current source via a series-connected pulsegenerator and resistor, a current integrator connected in parallel tothe resistor, and means for connecting the positive pole of the directcurrent source to the workpiece.
 2. An apparatus according to claim 1wherein the pulse generator supplies pulses between 5 and 20milliseconds.